The present invention relates to pressure monitoring devices and in particular to probes for use in monitoring the blood pressure of a patient.
The measurement of intravascular blood pressure is an important feature of modern health care. In the past intravascular blood pressure has been measured by inserting a catheter into a blood vessel, filling the catheter with a fluid and coupling the catheter by tubing to an external pressure transducer so that the blood pressure is transmitted hydraulically to the pressure transducer.
This type of apparatus has certain disadvantages, for example, when the apparatus is being set up, all traces of air must be meticulously flushed out after slow gravity filling of the apparatus with the hydraulic fluid. During monitoring the apparatus must be periodically checked for the presence of bubbles, which must be removed if found. The infusion of air bubbles into the patient's blood stream can represent a hazard to patient safety as a result of air emboli. Further, air bubbles in the apparatus will effect the accuracy of the apparatus.
Fibre optic pressure sensors offer several advantages over the conventional apparatus referred to above. For example, the small size of fibre optic sensors and their immunity to electromagnetic interference all obtained without recourse to special measures and their attendant costs make fibre optic sensing advantageous particular for in vivo medical applications.
It is known from U.S. Pat. No. 3,215,135 for a blood pressure measuring device to include means in the form of a housing for supporting a light reflecting diaphragm deforamable in response to the pressure of a fluid applied directly to one face of the diaphragm. Two flexible light guides extend into the housing and terminate at a location spaced from and adjacent the diaphragm. In use, one light guide conducts light from a source and emits it on to the opposite face of the diaphragm which reflects a proportional part of the light from the emitter towards a receiver light guide.
In use, when the diaphragm is brought into contact with the fluid the pressure of which is to be determined, the pressure of the fluid will deform the diaphragm and thus alter the intensity of light being received by the receiver light guide. By this means the pressure of the fluid can be determined.
A disadvantage of this known blood pressure measuring device is that there is only one emitter light guide and one receiver light guide positioned symmetrically relative to the reflective face of the diaphragm. This means that should there be any gross or localised reflectively change in the diaphragm this would result in a loss of accuracy. Furthermore, the light guides are in the form of optical fibre bundles which are bulky and relatively rigid.
In a paper entitled "Environmentally Insensitive Diaphragm Reflectance Pressure Transducer" by Christopher M. Lawson and V. J. Tekippe there is disclosed a fibre optic sensor that determines pressure from diaphragm curvature. Light is brought to a reflective surface of the diaphragm by a circle of fibres such that an annular concentric region on the diaphragm is illuminated. Light reflected from the diaphragm is then distributed among receiver fibres arranged concentrically with the emitter fibres both inside and outside of the emitter fibres. The diaphragm deflection is then derived from the ratio of the light received by the outside receiver fibres to that from the inside receiver fibres.
A disadvantage of the fibre optic sensor as described in this paper is that it includes a plurality of emitter fibre optic bundles and a plurality of receiver fibre optic bundles which makes the sensor bulky, relatively rigid and expensive to manufacture.